1. Field of the Invention
The present invention relates generally to prosthetic devices and more particularly to a permanent prosthethic ligament.
2. Description of the Prior Art
Recently, interest has increased in the development of prosthetic ligaments and tendons. One difficulty encountered in designing such prostheses arises from the strength and flexibility requirements for such devices. Due to the flexible nature of ligaments and tendons, prosthetic devices designed to replace natural ligaments and tendons tend to undergo plastic and constructional deformation, possibly resulting in a change in stiffness characteristics over time.
The goal of ligament replacement is to permit the recipient to return to his or her full range of activity as soon as possible. To that end, researchers have attempted by several means to mimic some of the parameters of strength, flexibility, extension and/or recovery found in natural ligaments. Natural ligaments are bands of flexible fibrous connective tissue which join bones or hold organs in place. The mechanical properties of a natural ligament were reported by D. Butler et al., "On the Interpretation of Our Anterior Cruciate Ligament Data", 196 CLINICAL ORTHOPAEDICS AND RELATED RESEARCH, SYNTHETIC LIGAMENTS AND TENDONS (H. Alexander & A. Weiss eds. June 1985), at 26. Butler et al. measured the stiffness of the anterior cruciate ligament-bone unit. The stiffness measured for young donors was 182.+-.56 kilonewtons per meter (1038.+-.0.32 lb/in) and for older donors was 129.+-.39 kilonewtons per meter (735.3.+-.0.16 lb/in).
The length of the adult anterior cruciate ligament (ACL) ranges between 27 to 39 mm (1.06-1.56 inches). The stiffness of the human ACL is reported to decrease roughly 30% with increasing age. Peak loads experienced by the ACL can range between 170 N (about 38 lbs.) for normal walking to 700 N (about 158 lbs.) for occasional jolts, such as tripping or jumping to one side. Under extreme loads, ranging between 501 to 1730 N (113-389 lbs.), the human ACL can tear. The maximum force tolerated for tissue obtained from young donors was 1730.+-.66 N (about 389.+-.15 lbs.). Extreme loads placed on prosthetic ligaments cause them to experience plastic deformation, an irreversible change in the microstructure of the device, a phenomenon often referred to as "creep". Prosthetic ligaments which have undergone creep are more susceptible to damage.
Laxity in the human ACL is measured clinically by means of the anterior drawer test (90.degree. flexion) or the Lachman test (20% flexion). It is reported that a knee exhibiting an anterior drawer greater than 10-15 mm (0.39-0.59 inch) under a load of 89 N (20 lbs.) requires repair. It is also reported that stable knees generally have an anterior drawer of less than 6-8 mm (0.23-0.31 inch); and that laxity relative to a patient's other healthy knee of greater than 2 mm (0.08 inch) indicates poor ACL support in the injured knee.
Often, torn or ruptured human ACL's are repaired with sutures, or augmented with autogenous tissue such as the patellar tendon. This type of repair requires many weeks of recovery. A prosthetic device should eliminate long term recovery and restore stability to the involved joint. However, it is important that creep and changes in stiffness of the prosthesis be minimized so that laxity does not return to the involved joint over time. A stiffness similar to that of the natural ligament is desirable.
Attempts to repair or replace damaged ligaments have been varied and generally inadequate for immediately restoring full strength and stability to the involved joint. Workers in the field have transferred natural tissue from other parts of a patient's body to the involved joint. Synthetic materials have also been used to augment natural tissue transfers. A number of techniques employing carbon fiber-type or polypropylene augmentation devices are described in CLINICAL ORTHOPAEDICS, supra. For example, a flat strap-like braid of polypropylene fibers was used to augment natural tissue grafts in studies conducted on goats. See G. McPherson et al., "Experimental Mechanical and Histologic Evaluation of the Kennedy Ligament Augmentation Device", CLINICAL ORTHOPAEDICS, supra at 186. The time required for the recipient to return to normal activity is generally about one year or longer.
As an alternative to natural tissue grafts, xenografts, tissue grafts from a species other than the recipient species, have been implanted to replace natural ligaments. Like the natural tissue grafts and the augmentation devices, xenografts have tended to be unpredictable in the long term for restoring full strength and stability to the involved joint.
Another type of prosthetic ligament relies on bone ingrowth to aid in the attachment of the ligament to bone. Bone ingrowth strengthens the attachment but requires about six months to complete. In the meantime, the recipient's mobility should be restricted. Dahlen et al. U.S. Pat. No. 4,187,558, which issued on Feb. 12, 1980, describes a flexible braid made of Dacron (polyethylene terephthalate), encased in silicone rubber. A velour covered collar at one or both ends of the braid aids in attachment to the bone and promotes bone ingrowth to anchor the device.
Several permanent, nonaugmented prosthetic ligaments have been developed. A permanent prosthesis is one which assumes its full strength initially upon implantation, is not intended to gradually resorb or disintegrate over time and does not depend on autographs or "regrown" natural ligament tissue for its ultimate success. Dore et al. U.S. Pat. No. 4,301,551, which issued on Nov. 24, 1981 describes a deformable silicone core surrounded by a tensionable wrapping of polymeric or stainless steel threads wound in a helical angle about the core. The core is the load bearing member and is capable of large elastic deformation in response to compression by the threads when the device is stretched. Two rigid plastic or stainless steel rods, one at each end of the core, connect the device to the bones of a joint.
Treace U.S. Pat. No. 3,953,896, which issued on May 4, 1976 describes a prosthetic ligament made of a flexible, ultra high molecular weight polyethylene rod. Stainless steel sleeves and polyethylene nuts on each end of the flexible rod hold the prosthetic ligament to the bones.
A third permanent, nonaugmented prosthetic ligament reported by C. Bolton and W. Bruchman, "The GORE-TEX.TM. Expanded Polytetrafluoroethylene Prosthetic Ligament", CLINICAL ORTHOPAEDICS, supra at 202, is constructed of bundles of Gore-Tex.RTM. fibers arranged in a braided configuration. The braid is fixed by bone screws placed through eyelets at each end of the braid. According to a description of the Gore-Tex.RTM. ligament in a product brochure published by the manufacturer, the ultimate tensile strength of the device is approximately 1160 lbs. The product brochure indicates that prior to packaging, each Gore-Tex.RTM. ligament is tested to 1000 lbs. as a quality control measure.
Testing of a prosthesis composed of a continuous weave of Dacron fibers in the form of a braid with a hollow central core gave poor results. The investigators concluded, as reported in A. Ellison, "Studies on the Prosthetic Replacement of the Anterior Cruciate Ligament" (thesis to American Orthopaedic Association, Jan. 1, 1976), that Dacron will never be satisfactory for an anterior cruciate ligament prosthesis.
Zachariades U.S. Pat. No. 4,587,163 discloses an isotropic semicrystalline morphology of ultra high molecular weight polyethylene for use in making artificial tendons or ligament protheses. The artificial tendon can be sutured to a natural tendon segment. The polyethylene described by Zachariades is an ultradrawn melt crystallized ribbon-like structure.
A variety of means are disclosed to attach prosthetic ligaments to the appropriate body member. Bokros U.S. Pat. No. 4,149,277 discloses a ligament or tendon prosthesis which includes a loop at the bone joining end of a braided strand. The Dore patent referenced above employs rigid plastic or stainless steel rods and the Treace patent employs stainless steel sleeves and polyethylene nuts. The Gore-Tex.RTM. ligament includes eyelets at each end to connect the ligament to bone.
As mentioned above, one problem often experienced by recipients of prosthetic ligaments is that the braided ligaments undergo constructional deformation after implant and, as a result, becomes too lax over time or after the first load is placed on the joint. Constructional deformation occurs when the fibers of the braided ligament prosthesis compact and undergo helical changes. There is a certain amount of "slack" in the prosthetic ligaments heretofore available which permits them to undergo constructional deformation with use. The prosthetic ligament lengthens and loses its tensioning capacity for the involved joint.
An object of the present invention is to provide a permanent nonaugmented prosthetic ligament having parameters of strength and flexibility that at least approximate those of a natural ligament. A further object of the present invention is to provide such a prosthetic ligament that does not depend solely upon bone ingrowth for strengthening the attachment to the bones of the involved joint, and thus, does not require long periods of recuperation before the recipient can resume a full range of normal activity. It is a further object of the present invention to provide a permanent ligament prosthesis which will minimize any observed increase in laxity with time and will undergo minimal constructional deformation after the first shock load.